We consider a recently discovered class of instabilities, driven by
cosmic ray streaming, in a variety of environments. We show that
although these instabilities have been discussed primarily in the
context of supernova-driven interstellar shocks, they can also operate
in the intergalactic medium and in galaxies with weak magnetic fields,
where, as a strong source of helical magnetic fluctuations, they could
contribute to the overall evolution of the magnetic field. Within the
Milky Way, these instabilities are strongest in warm ionized gas and
appear to be weak in hot, low density gas unless the injection
efficiency of cosmic rays is very high.